Automatic injection devices are routinely used in the medical field to deliver a measured dose of medicine to a user. Due to their user friendly design, they can be safely used by patients for self-administration, although in some circumstances they may be used by trained personnel.
A typical automatic injection device comprises several parts which may include; a syringe containing medicine, a needle fixed to the end of the syringe, a firing mechanism including a spring (or possibly other drive means such as an electric motor or gas drive means), and a trigger. The spring may be preloaded, or may be set using a dose setting mechanism such as a dial. The firing mechanism is activated by the trigger and forces the medicine through the needle and into the user. A mechanical lock may be provided to prevent the trigger from being accidentally pressed. This could be, for example, simply a catch that must be moved out of the way in order to access the trigger.
Single use, disposable automatic injection devices are delivered to end users in an assembled state, with a medicine syringe contained within the device housing and a needle fixed to the end of the syringe. In order to ensure sterility of the needle, the projecting end of the needle is contained within an rubber or elastomer “boot”. Typically, the boot forms an interference fit around the narrowed end portion of the syringe body. The tip of the needle may penetrate the end of the boot. In the case of re-useable automatic injection devices, an end user must typically open the housing and press a new single-use syringe into position. The single-use syringe will have a needle and boot already in place.
The injection device may also comprise a boot remover to allow the end user to easily and safely remove the boot and thereby expose the needle. Typically, the boot remover is fitted around or inside the proximal end of the device prior to insertion of the syringe into the housing. When the syringe is pressed into the housing, the boot protecting the needle is captured by the boot remover, i.e. snaps into place within the boot remover. A needle shield may be further provided around the needle, such that the needle remains protected even after the boot has been removed. This is relevant to so-called “auto-injectors” which, in addition to driving the medicine through the needle, perform an initial step of inserting the needle through the skin using the force provided by the injection spring (or possible a secondary spring).
When a single use automatic injection device is to be used, a user should first remove the boot remover and boot to expose the needle. NB. the needle remains surrounded by the needle shield at least in the case of an auto-injector. The user will then release the mechanical lock, such that the trigger can be pressed. The user can then place the auto-injector against the surface of the skin and press the trigger to push the needle through the skin and force the medicine through the needle. In the case of an auto-injector, a carriage and carriage-return spring may cause the needle to be returned to a position within the needle shield.
A problem with single use automatic injection devices occurs when a user forgets to first remove the boot, and, instead, operates the trigger with the boot still in place. This is particularly likely in the case of an auto-injector, where the needle and boot are not readily visible. If the boot is not removed before firing, no drug is delivered to the user. Furthermore, since the medicine will now be under pressure, there is a risk that the user may inadvertently empty the syringe contents into the air if, when realising their error, they subsequently remove the boot.
A user may not have an abundance of medicine and so waste may be a serious issue. Waste may also be undesirable due to cost implications: some medicines can be extremely expensive. Therefore, there exists a need to provide an automatic injection device that overcomes the problem of a device being fired prior to removal of a boot.